The invention described herein was made by an employee of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor.
This invention is in the field of micro electromechanical devices or related materials.
Strain gages have been bonded on metal diaphragms to produce pressure sensors or accelerometers. Because these transducers are made of materials with dissimilar properties, they suffer from coefficient of thermal expansion (CTE) mismatch, which leads to fatigue and early failure. In addition the production process is time consuming since each strain gage must be placed on the diaphragm one at a time.
I am a named inventor of U.S. Pat. No. 5,637,905 to Carr et al. and it discloses a high temperature pressure and displacement microsensor made from a Si substrate. A first coil structure is positioned within a recess in the Si and a pressure diaphragm is glass bonded about the periphery to the rim of the semiconductor substrate. A second coil structure is positioned on the underside of the pressure diaphragm and is electrically isolated from the first coil structure. The coils are inductively coupled together and provide an output indicative of changes in the coupling between the coils.
My U.S. Pat. No. 6,248,646 discloses a process for making an array of SiC wafers on standard larger industry sized wafers. This patent discusses the operating conditions for SiC and SiC-On-Insulator technology and cites the need for sensors made from SiC.
U.S. Pat. No. 5,447,067 to Biebl et al. discloses an acceleration sensor constructed on Silicon-On-Insulator substrate. Piezoresistors are disclosed for use in conjunction with a proof mass suspended by one or more resilient elements. These sensors are not useable in harsh environments. U.S. Pat. No. 5,576,250 to Diem et al. discloses a process for the production of accelerometers using Silicon-On-Insulator technology. The ""250 patent discloses an accelerometer with moving elements consisting of one or more flexible beams supporting a seismic mass. Further, the ""250 patent discloses packaging of accelerometers and the driving circuit by multichip module technology.
Sensors manufactured from 3C-SiC, 4H-SiC and 6H-SiC are used in harsh environments, for example high temperature environments, high vibration environments, radiation environments and corrosive environments. xe2x80x9cHxe2x80x9d means hexagonal and xe2x80x9cCxe2x80x9d as used in xe2x80x9c3Cxe2x80x9d means Cubic and both refer to the crystalline structure of SiC.
SiC is a wide band gap semiconductor. Semiconductors are materials whose electrical conductivity is between that of a conductor and that of an insulator. xe2x80x9cIf an electron in an atom happens to be in an energy level which overlaps a higher, empty level, that electron proves to be essentially free from its original atom. It is then capable of moving freely through the solid, and the material will be a conductor, i.e., a metal. However, if the electron in the highest energy state of the atom exists in a level which does not overlap higher energy levels, this electron will be firmly held to its atom. Such a material will be a nonconductor of electricity. An intermediate case exists if the energy levels do not overlap but are close enough so that the energy gap between them is of the order of thermal energies. These materials are called semiconductors.xe2x80x9d Introduction To Physics For Scientists And Engineers, Copyright 1969, McGraw-Hill, Inc., Library of Congress Catalog Card Number 69-13598, ISBN 07-008833-0, pgs. 804-805.
The semiconductor SiC is known as a wide band gap semiconductor meaning that electrons in the valence band must traverse an energy gap of several electron Volts (eV) at 300 K to reach the conduction band. SiC is operable at temperatures up to 873 K without substantial leakage current. Leakage current, for example that is due to the temperature of the operating environment, is kept to a minimum in SiC.
Batch fabrication of a single function type SiC sensors, namely, pressure sensors, has been demonstrated and has piqued the interest of many who desire stable sensors operable in harsh environments. SiC is, however, a very expensive material with wafer costs much greater than conventional silicon semiconductor for a two inch diameter wafer. One such wafer can produce between 100-400 pressure sensors.
There is not enough demand, however, for batch production of pressure sensors alone. Unlike silicon based sensors, silicon carbide sensors have a low volume specialized market The current process for fabricating silicon carbide sensors is limited to producing only one type of sensor per wafer at a time and, as such, the commercial viability of silicon carbide is greatly reduced. Further, there is no known process for simultaneously making different devices (sensors) having different functionality at the same time. Several different types of sensors exist such as accelerometers having proof masses suspended therein and pressure sensors having diaphragms.
There is a need for SiC accelerometers having suspended proof masses. Presently, such devices are not manufactured and are not believed to exist. Further, there is a need for the batch fabrication of multifunctional, multistructural sensors and other devices manufactured from SiC.
Although batch fabrication of SiC pressure sensors has been demonstrated, the economic viability of SiC sensors heretofore has been in doubt because there is no need for the mass production of one type of sensor, i.e, a pressure sensor. Industry remains reluctant to devote its resources to commercial production of SiC sensors for the following reasons:
(1) unlike Si sensors, SiC sensors of any one particular type have a low volume, specialized market;
(2) SiC has an inherently high material and capital cost when only one sensor is made in bulk from a single wafer and as a result the profitability incentive does not exist to encourage commercial production; and,
(3) the current process for fabricating these devices is limited to producing only one type of device at a time therefore doubling the fabrication cost for making two different devices.
One major problem in the batch manufacturing of SiC multistructural sensors is that some of the sensors such as accelerometers require the construction of apertures or annular recesses in the substrate. An aperture or a recess is three dimensional. Sensors such as accelerometers desirably include a suspended mass in the substrate from which they are manufactured. This mass is made from SiC and the piezoresistance of the n-type or p-type SiC which connects the mass to the remainder of the substrate is measured. Mathematical analysis of the piezoresistance determines the value and direction of acceleration. The suspension of the mass requires that the substrate be etched very precisely.
It is not possible to precisely construct the apertures or annular recesses in the SiC substrate before metallization because they interfere adversely with the remaining fabrication/manufacture of the SiC sensor. SiC sensors precisely measure the piezoresistance of specific areas of n-type SiC and, therefore, it is necessary that the contact metallization be precisely located and engage the n-type SiC in those specific areas. Positioning of the contact metallization is controlled by a masking process where photoresist is spun onto the wafer that is held under suction on a chuck. Therefore, if the wafer was perforated prior to application of the photoresist it would not be possible to create a suction due to the perforations. Further, the suction from the chuck through the perforations in the wafer would disturb the uniform application of the photoresist. SiC wafers are rotated between 1000 to 7000 revolutions per minute as photoresist is applied to the center of the wafer. As photoresist is spread radially it will impact whatever three dimensional apertures or recesses exist and will not spread evenly in those areas thereby resulting in low yield of the wafer. By low yield, it is meant that many sensors will be defective due to poor patterning of the photoresist. At costs approaching $3,500 for a two inch diameter SiC wafer with epilayer, it is important that its use be maximized. It is desired that approximately 100-400 sensors be generated from each wafer so as to maximize the economy of volume and batch production of the sensors.
A better understanding of the invention will be had when reference is made to the SUMMARY OF THE INVENTION, BRIEF DESCRIPTION OF THE DRAWINGS, DESCRIPTION OF THE INVENTION and CLAIMS which follow hereinbelow.
The simultaneous fabrication of multi-functional SiC micro electromechanical devices is disclosed, claimed and described herein. Simultaneous fabrication of flow sensors, pressure sensors, accelerometers, inertial sensors, angular rate sensors and yaw rate sensors from SiC is accomplished by this invention. These sensors may be configured as desired by the particular user for the user""s specific application. The instant invention allows for the simultaneous production of SiC sensors of different types from the same wafer thus greatly increasing the viability of SiC for use as sensors.
Substrates comprising other materials are specifically contemplated by this invention. AlN, BC, BN, and Al2O3 may, for example be used. Any substrate upon which an epilayer may be grown is contemplated to be within the scope of this invention.
This invention offers a global platform for bulk micro machining process in SiC or in any one of several other material mentioned above. It offers various manufacturers the opportunity to simultaneously produce multifunctional products on a single SiC wafer (or wafer made from another material) and thereby greatly lower capital equipment and production cost.
The sensing principle utilizes the piezoresistance of the single crystal SiC or other material. Piezoresistance indicates a dependence of resistivity on mechanical strain. In particular, the instant invention by way of example utilizes the piezoresistance of the n- or p-type epilayer of a SiC wafer. The low resistivity n-type epilayer, in effect, acts as a variable resistor which is mounted atop a high resistivity p-type SiC substrate. It is the mechanical deformation of this n-type epilayer which causes resistivity changes which are measured by applying a voltage differential across a portion of the n- (or, p-) type epilayer. As the resistance changes as a function of mechanical deformation of the n-type epilayer, the flow of electrical current through the n-type epilayer will change for a given voltage. The instant invention discloses a novel SiC sensor as well as a method for bulk manufacturing of multifunction SiC sensors. The examples given for the bulk manufacturing of SiC sensors are equally applicable to sensors made from the other materials mentioned above.
One major factor in bulk micromachined SiC sensors is the presence of three dimensional structures. It is difficult to apply a planar coating of photoresist if three dimensional apertures or recesses exist in the substrate. To overcome this barrier, this invention employs a process flow reversal whereby contact metal is first sputter deposited onto the n-type epilayer of the SiC wafer before recesses or apertures are etched into or through the wafer. Additionally, if and once holes are pierced through the wafer it is extremely difficult to hold the wafer in a vacuum chuck. Aluminum is deposited by electron beam evaporation (e-beam evaporation) onto the entire planar surface of the contact metal. Photoresist is spun on the substantially planar contact metal and then masked and exposed under ultraviolet light. Photoresist imidized under such exposure is stripped away with developer and then the unwanted Aluminum is etched with TMAH (Trimethyl Ammonium Hydroxide). Next, the metal(s) (in this case layers of Platinum, Tantalum Disilicide and Titanium) is/are dry etched using the Aluminum as the etch mask.
Next, photoresist is spun onto the remaining Aluminum and the oxide layer on the n-type epilayer. Another mask is applied, exposed under ultraviolet light and the imidized photoresist is stripped away after developing and the unimidized photoresist is left behind. Next Indium Tin Oxide (ITO or Nickel (Ni)) is sputter deposited on the Aluminum and the unimidized photoresist. The ITO, however, does not completely cover the unimidized photoresist enabling Acetone to dissolve the unimidized photoresist when submersed in Acetone. This process lifts off ITO (or Nickel) on the photoresist. Now with the oxide exposed and the remainder of the wafer protected by the ITO (or Nickel), deep reactive ion etching occurs and recesses or apertures may be formed in the SiC substrate. This process may be used to produce a suspended proof mass in the SiC wafer. The proof mass may move out of the plane in which it resides at rest. Recesses in any shape or form may be created in the SiC wafer using these techniques. The ITO and the remaining Aluminum is removed with hot phosphoric acid. Wires can then be attached to the contact metal such as Platinum. Other metals may be used for the contact Depending on the configuration of the sensor desired, the back side of the SiC wafer may be etched to provide a diaphragm.
Accordingly, it is an object of the present invention to provide an accelerometer made from SiC, for example, or from any material used as a substrate upon which an epilayer may be grown or deposited. It is a further object of the present invention to provide a method of producing an accelerometer made from SiC. Further, it is a further object of the present invention to provide a method of simultaneously making a plurality of multifunctional sensors from SiC, for example, or from any material used as a substrate upon which an epilayer may be grown or deposited. It is a further object of the present invention to manufacture a plurality of similar or diverse sensors simultaneously by a process which includes the step of first applying contact metal to engage the SiC and to sense resistance changes of the SiC in response to mechanical deformation of the SiC. It is a further object of the present invention to provide SiC sensors capable of operating in harsh environments.